Lithium composite compound particles and process for producing the same, and non-aqueous electrolyte secondary battery

ABSTRACT

The present invention relates to lithium composite compound particles having a composition represented by the formula: Li 1+x Ni 1-y-z Co y M z O 2  (M=B or Al), wherein the lithium composite compound particles have an ionic strength ratio A (LiO − /NiO 2   − ) of not more than 0.3 and an ionic strength ratio B (Li 3 CO 3   + /Ni + ) of not more than 20 as measured on a surface of the respective lithium composite compound particles using a time-of-flight secondary ion mass spectrometer. The lithium composite compound particles of the present invention can be used as a positive electrode active substance of a secondary battery which has good cycle characteristics and an excellent high-temperature storage property.

TECHNICAL FIELD

The present invention relates to lithium composite compound particleswhich are capable of exhibiting good cycle characteristics and anexcellent high-temperature storage property when used as a positiveelectrode (cathode) active substance of secondary batteries, and asecondary battery using the lithium composite compound particles.

BACKGROUND ART

With the recent rapid development of portable and cordless electronicdevices such as audio-visual (AV) devices and personal computers, thereis an increasing demand for secondary batteries or cells having a smallsize, a light weight and a high energy density as a power source fordriving these electronic devices. Also, in consideration of globalenvironments, electric cars and hybrid cars have been recently developedand put into practice, so that there is an increasing demand for lithiumion secondary batteries for large size applications having excellentstorage characteristics. Under these circumstances, the lithium ionsecondary batteries having advantages such as a large charge/dischargecapacity and good storage characteristics have been noticed.

Hitherto, as positive electrode active substances useful for highenergy-type lithium ion secondary batteries having a 4 V-grade voltage,there are generally known LiMn₂O₄ having a spinel structure, LiMnO₂having a zigzag layer structure, LiCoO₂ and LiNiO₂ having a layerrock-salt structure, or the like. Among these secondary batteries usingthese active substances, lithium ion secondary batteries using LiNiO₂have been noticed because they have a large charge/discharge capacitythereof. However, these materials tend to be deteriorated in thermalstability upon charging and charge/discharge cycle durability, and,therefore, it has been required to further improve properties thereof.

One of factors causing deterioration of characteristics of the positiveelectrode active substances is considered to reside in impurities whichare present on the surface of the respective particles. That is, when anexcess amount of lithium is present on the surface of the particles uponsynthesis thereof, undesirable gelation tends to be caused when formingan electrode therefrom. In addition, when the excess amount of lithiumis subjected to carbonation, generation of a carbon dioxide gas tends tobe undesirably caused owing to a reaction within the battery, so thatthe battery tends to suffer from swelling, resulting in deterioratedcharacteristics of the battery. Further, if sulfates or the like arepresent on the particles, undesirable increase in resistance value ofthe battery tends to be caused upon storage.

To solve the above conventional problems, it has been strongly requiredthat the amount of impurities which are present on the surface of theparticles is reduced to control the surface condition of the particles,so that side reactions within the battery upon charging and dischargingare suppressed, and the particles and the electrode are prevented frombeing deteriorated in their characteristics to improve cyclecharacteristics and high-temperature storage property of the resultingbattery.

Conventionally, for the purpose of improving various characteristics ofthe secondary battery, there are known the techniques for improving acapacity of the secondary battery (Patent Documents 1 to 7), thetechniques for improving cycle characteristics of the secondary battery(Patent Documents 8 to 10), the techniques for improving a storageproperty of the secondary battery (Patent Documents 3 and 11), and thetechniques for improving a thermal stability of the secondary battery(Patent Documents 5 to 7 and 12) or the like.

-   Patent Document 1: Japanese Patent Application Laid-open (KOKAI) No.    3-64860-   Patent Document 2: Japanese Patent Application Laid-open (KOKAI) No.    9-259879-   Patent Document 3: Japanese Patent Application Laid-open (KOKAI) No.    2003-17054-   Patent Document 4: Japanese Patent Application Laid-open (KOKAI) No.    2004-171961-   Patent Document 5: Japanese Patent Application Laid-open (KOKAI) No.    2007-273106-   Patent Document 6: Japanese Patent Application Laid-open (KOKAI) No.    2008-117729-   Patent Document 7: Japanese Patent Application Laid-open (KOKAI) No.    2008-198363-   Patent Document 8: Japanese Patent Application Laid-open (KOKAI) No.    4-328277-   Patent Document 9: Japanese Patent Application Laid-open (KOKAI) No.    8-138669-   Patent Document 10: Japanese Patent Application Laid-open (KOKAI)    No. 9-17430-   Patent Document 11: Japanese Patent Application Laid-open (KOKAI)    No. 9-231963-   Patent Document 12: Japanese Patent Application Laid-open (KOKAI)    No. 2007-273108

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

At present, it has been strongly required to provide a positiveelectrode (cathode) active substance capable of fulfilling the aboveproperties. However, such a positive electrode active substance has notbeen obtained until now.

That is, it has been attempted to improve a capacity, cyclecharacteristics, a storage property and a thermal stability of thesecondary battery by washing particles as the positive electrode activesubstance with water and reducing an amount of impurities which arepresent on the surface thereof.

In the above method in which excessive lithium is washed out from thesurface of the particles, it will be expected to improve a coatabilityof the particles and suppress occurrence of side reactions within thesecondary battery.

However, when the water-washing conditions are inadequate, lithium tendsto be released from an inside of the positive electrode activesubstance, so that an inherent crystal structure thereof tends to bebroken, and the resulting secondary battery tends to be deteriorated incycle characteristics.

Under these circumstances, an object of the present invention is toprovide lithium composite compound particles as a positive electrodeactive substance which are well controlled in a crystal structure andamount of impurities present on the surface of the respective particlesso as to improve cycle characteristics and a storage property of asecondary battery produced using the particles.

Means for Solving the Problem

The above object and technical task can be achieved by the followingaspects of the present invention.

That is, according to the present invention, there are provided lithiumcomposite compound particles comprising a lithium composite compoundrepresented by the following compositional formula 1, which lithiumcomposite compound particles have an ionic strength ratio A (LiO⁻/NiO₂⁻) of not more than 0.3 and an ionic strength ratio B (Li₃CO₃ ⁺/Ni⁺) ofnot more than 20 as measured on a surface of the respective lithiumcomposite compound particles using a time-of-flight secondary ion massspectrometer (Invention 1):

Li_(1+x)Ni_(1-y-z)Co_(y)M_(z)O₂  Compositional formula 1

wherein M is at least one element selected from the group consisting ofB and Al; and x, y and z satisfy −0.02≦x≦0.02, 0<y≦0.20 and 0<z≦0.10,respectively.

Also, according to the present invention, there are provided the lithiumcomposite compound particles as described in the above Invention 1,wherein the lithium composite compound particles have an averagesecondary particle diameter of 1 to 30 μm (Invention 2).

Also, according to the present invention, there are provided the lithiumcomposite compound particles as described in the above Invention 1 or 2,wherein the lithium composite compound particles have a powder pH valueof not more than 11.0 as measured in a 2% by weight suspension preparedby dispersing the lithium composite compound particles in water(Invention 3).

Also, according to the present invention, there are provided the lithiumcomposite compound particles as described in any one of the aboveInventions 1 to 3, wherein the lithium composite compound particles havea carbon content of not more than 300 ppm (Invention 4).

Also, according to the present invention, there are provided the lithiumcomposite compound particles as described in any one of the aboveInventions 1 to 4, wherein the lithium composite compound particles havea sulfur content of not more than 100 ppm, an ionic strength ratio C(LiSO₃ ⁻/NiO₂ ⁻) of not more than 0.3 and a sodium content of not morethan 100 ppm (Invention 5).

Also, according to the present invention, there are provided the lithiumcomposite compound particles as described in any one of the aboveInventions 1 to 5, wherein the lithium composite compound particles havea lithium carbonate component content of not more than 0.30% by weightand a lithium hydroxide content of not more than 0.30% by weight(Invention 6).

Also, according to the present invention, there are provided the lithiumcomposite compound particles as described in any one of the aboveInventions 1 to 6, wherein the lithium composite compound particles havea specific surface area of 0.05 to 0.7 m²/g (Invention 7).

In addition, according to the present invention, there is provided aprocess for producing the lithium composite compound particles asdescribed in any one of the above Inventions 1 to 7, comprising thesteps of (1) treating lithium composite compound particles with a watersolvent to remove impurities therefrom; and (2) subjecting the lithiumcomposite compound particles obtained in the step (1) to heat treatment,

a ratio of a total molar amount of lithium to a total molar amount of atransition element, aluminum and boron in the lithium composite compoundparticles used in the step (1) being not less than 1.02 and not morethan 1.10 (Invention 8).

Also, according to the present invention, there is provided the processas described in the above Invention 8, wherein the heat treatment in thestep (2) is conducted in a temperature range of 500 to 850° C. in an airor oxygen atmosphere having a carbonate concentration of not more than100 ppm (Invention 9).

Further, according to the present invention, there is provided anon-aqueous electrolyte secondary battery comprising the lithiumcomposite compound particles as described in any one of the aboveInventions 1 to 7 (Invention 10).

Effect of the Invention

The lithium composite compound particles of the present invention areexcellent in cycle characteristics and high-temperature storage propertywhen used as a positive electrode active substance of a secondarybattery, and therefore can be suitably used as a positive electrodeactive substance of a secondary battery.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

The construction of the present invention is described in detail below.

First, the lithium composite compound particles according to the presentinvention are described.

The lithium composite compound particles according to the presentinvention have a composition represented by the following compositionalformula 1:

Li_(1+x)Ni_(1-y-z)Co_(y)M_(z)O₂  Compositional formula 1

wherein M is at least one element selected from the group consisting ofB and Al; and x, y and z satisfy −0.02≦x≦0.02, 0<y≦0.20 and 0<z≦0.10,respectively.

The suffixes x, y and z are more preferably −0.015≦x≦0.015, 0.001≦y≦0.18and 0.001≦z≦0.09, respectively, and still more preferably −0.01≦x≦0.01,0.01≦y≦0.15 and 0.01≦z≦0.08, respectively.

The lithium composite compound particles according to the presentinvention have an ionic strength ratio A (LiO⁻/NiO₂ ⁻) of not more than0.3 as measured on a surface of the respective lithium compositecompound particles using a time-of-flight secondary ion massspectrometer. When the ionic strength ratio A (LiO⁻/NiO₂ ⁻) of thelithium composite compound particles is more than 0.3, the secondarybattery produced using the lithium composite compound particles tends tobe deteriorated in cycle characteristics. The ionic strength ratio A(LiO⁻/NiO₂ ⁻) of the lithium composite compound particles is preferably0.01 to 0.25.

The lithium composite compound particles according to the presentinvention have an ionic strength ratio B (Li₃CO₃ ⁺/Ni⁺) of not more than20 as measured on a surface of the respective lithium composite compoundparticles using a time-of-flight secondary ion mass spectrometer. Whenthe ionic strength ratio B (Li₃CO₃ ⁺/Ni⁺) of the lithium compositecompound particles is more than 20, the secondary battery produced usingthe lithium composite compound particles tends to be deteriorated incycle characteristics. The ionic strength ratio B (Li₃CO₃ ⁺/Ni⁺) of thelithium composite compound particles is preferably 0.1 to 19.0.

The lithium composite compound particles according to the presentinvention preferably have an ionic strength ratio C (LiSO₃ ⁻/NiO₂ ⁻) ofnot more than 0.3 as measured on a surface of the respective lithiumcomposite compound particles using a time-of-flight secondary ion massspectrometer. When the ionic strength ratio C (LiSO₃ ⁻/NiO₂ ⁻) of thelithium composite compound particles is more than 0.3, the secondarybattery produced using the lithium composite compound particles tends tobe deteriorated in storage property. The ionic strength ratio C (LiSO₃⁻/NiO₂ ⁻) of the lithium composite compound particles is more preferably0.01 to 0.25.

The lithium composite compound particles according to the presentinvention preferably have an average secondary particle diameter of 1.0to 30 μm. When the average secondary particle diameter of the lithiumcomposite compound particles is less than 1.0 μm, the resultingparticles tend to be undesirably lowered in packing density or increasedin reactivity with an electrolyte solution. When the average secondaryparticle diameter of the lithium composite compound particles is morethan 30 μm, the resulting particles tend to be deteriorated inconductivity owing to increase in diffusion distance of lithium ions,and the secondary battery produced using the particles tends to bedeteriorated in cycle characteristics, thereby failing to attain theaimed effects of the present invention. The average secondary particlediameter of the lithium composite compound particles is more preferably2.0 to 20 μm.

The lithium composite compound particles according to the presentinvention preferably have an average primary particle diameter of notless than 0.1 μm. When the average primary particle diameter of thelithium composite compound particles is less than 0.1 μm, the resultingparticles tend to be deteriorated in crystallizability, also resultingin deteriorated cycle characteristics of the secondary battery producedusing the particles. When the average primary particle diameter of thelithium composite compound particles is more than 15 μm, the resultingparticles tend to suffer from poor diffusion of lithium therein, so thatthe secondary battery produced using the particles also tends to bedeteriorated in cycle characteristics. The average primary particlediameter of the lithium composite compound particles is more preferably0.1 to 15 μm and still more preferably 0.5 to 12 μm.

The lithium composite compound particles according to the presentinvention preferably have a powder pH value (a pH value of water inwhich the particles are dispersed) of not more than 11.0. When thepowder pH value of the lithium composite compound particles is more than11.0, the positive electrode obtained therefrom tends to be deterioratedin coatability, and the secondary battery produced using the particlesalso tends to be deteriorated in cycle characteristics and storageproperty. The powder pH value of the lithium composite compoundparticles is more preferably not more than 10.8 and still morepreferably not more than 10.7. Meanwhile, the lower limit of the powderpH value of the lithium composite compound particles is usually 9.0.

The lithium composite compound particles according to the presentinvention preferably have a carbon content of not more than 300 ppm.When the carbon content of the lithium composite compound particles ismore than 300 ppm, the secondary battery produced using the lithiumcomposite compound particles tends to be deteriorated in cyclecharacteristics. The carbon content of the lithium composite compoundparticles is more preferably 1.0 to 250 ppm.

The lithium composite compound particles according to the presentinvention preferably have a sulfur content of not more than 100 ppm.When the sulfur content of the lithium composite compound particles ismore than 100 ppm, the secondary battery produced using the lithiumcomposite compound particles tends to be deteriorated in storageproperty. The sulfur content of the lithium composite compound particlesis more preferably not more than 50 ppm.

The lithium composite compound particles according to the presentinvention preferably have a sodium content of not more than 100 ppm.When the sodium content of the lithium composite compound particles ismore than 100 ppm, the secondary battery produced using the lithiumcomposite compound particles tends to be deteriorated in cyclecharacteristics. The sodium content of the lithium composite compoundparticles is more preferably not more than 50 ppm.

The lithium composite compound particles according to the presentinvention preferably have a lithium carbonate component content of notmore than 0.30% by weight. When the lithium carbonate content of thelithium composite compound particles is more than 0.30% by weight, thesecondary battery produced using the lithium composite compoundparticles tends to be deteriorated in cycle characteristics owing tooccurrence of side reactions and generation of gases within thesecondary battery. The lithium carbonate content of the lithiumcomposite compound particles is more preferably not more than 0.25% byweight.

The lithium composite compound particles according to the presentinvention preferably have a lithium hydroxide content of not more than0.30% by weight. When the lithium hydroxide content of the lithiumcomposite compound particles is more than 0.30% by weight, the positiveelectrode obtained from the lithium composite compound particles tendsto be deteriorated in coatability, and the secondary battery producedusing the lithium composite compound particles tends to be deterioratedin cycle characteristics. The lithium hydroxide content of the lithiumcomposite compound particles is more preferably not more than 0.20% byweight.

The lithium composite compound particles according to the presentinvention preferably have a BET specific surface area of 0.05 to 0.7m²/g. When the BET specific surface area of the lithium compositecompound particles is less than 0.05 m²/g, the secondary batteryproduced using the lithium composite compound particles tends to bedeteriorated in cycle characteristics. When the BET specific surfacearea of the lithium composite compound particles is more than 0.7 m²/g,the secondary battery produced using the lithium composite compoundparticles tends to be deteriorated in storage property. The BET specificsurface area of the lithium composite compound particles is morepreferably 0.06 to 0.6 m²/g.

Next, the process for producing the lithium composite compound particlesaccording to the present invention is described.

The lithium composite compound particles according to the presentinvention can be produced by conducting the step (1) of deaggregatinglithium composite compound particles previously prepared and thendispersing the thus deaggregated lithium composite compound particles inwater to wash the particles with the water, thereby removing impuritiestherefrom; and the step (2) of subjecting the lithium composite compoundparticles obtained in the step (1) to drying and then to heat treatmentin a temperature range of 500 to 850° C. in atmospheric air having acarbonate concentration of not more than 100 ppm or in oxygen having acarbonate concentration of not more than 100 ppm.

In the present invention, the lithium composite compound particles to beinitially treated in the above process may be produced by an ordinarymethod. For example, the lithium composite compound particles may beproduced by any of the method in which a lithium compound, a nickelcompound, a cobalt compound, an aluminum compound and/or a boroncompound are mixed with each other, and then the resulting mixture issubjected to heat treatment to thereby obtain the lithium compositecompound particles as aimed; the method in which a composite compoundcomprising nickel, cobalt, aluminum and/or boron is previously formedand then mixed with a lithium compound, and the resulting mixture issubjected to heat treatment; and the method in which a lithium compound,a nickel compound, a cobalt compound, an aluminum compound and/or aboron compound are reacted with each other in a solution thereof.

Meanwhile, the lithium composite compound particles to be initiallytreated preferably have a ratio of a total molar amount of lithium to atotal molar amount of a transition element (such as Co and Ni), aluminumand boron (Li/(Co+Ni+Al+B)) of not less than 1.02 and not more than1.10. When the above ratio is less than 1.02, the resulting lithiumcomposite compound particles tend to be deteriorated in capacity owingto an insufficient reaction between these elements. When the above ratiois more than 1.10, an excess amount of lithium tends to undesirablyremain as a residue. The ratio (Li/(Co+Ni+Al+B)) in the lithiumcomposite compound particles is more preferably 1.03 to 1.08.

In the present invention, the lithium composite compound particles arepreferably subjected to deaggregation before being washed with water.

In the present invention, the lithium composite compound particles aresuspended in deionized water which is used in an amount not less than 5times an amount of the lithium composite compound particles in terms ofa weight ratio therebetween and maintained at a temperature of nothigher than 20° C., over about 20 min, and the resulting suspension isfiltered and then washed with flowing deionized water which is used inthe same amount as that used upon the suspension. The suspension time ispreferably within 30 min.

The lithium composite compound particles thus washed with water aresubjected to filtration, drying and then heat treatment. When the amountof deionized water used for the washing is too small, the washing of thelithium composite compound particles tends to be insufficient. When thesuspension time is too long, the productivity of the lithium compositecompound particles tends to be undesirably lowered, and further Li tendsto be undesirably released from crystals of the particles. When thetemperature of deionized water used for the water-washing is too high,release of Li from the particles tends to occur very early, so that whenthe particles are washed with water to remove a surplus amount of Litherefrom, an additional amount of Li tends to be simultaneouslyreleased from crystals thereof. Therefore, it may be difficult to wellcontrol the composition of the resulting particles. As a result ofconsidering the above viewpoints, the water-washing is preferablyconducted using deionized water having a temperature of not higher than20° C. and preferably not higher than 10° C. within 20 min.

In order to stabilize a crystallinity of the surface of the respectiveparticles, it is required to subject the lithium composite compoundparticles to heat treatment. The heat treatment temperature is 500 to850° C. When the heat treatment temperature is lower than 500° C., thesecondary battery produced using the resulting lithium compositecompound particles tends to be deteriorated in storage property. Whenthe heat treatment temperature is higher than 850° C., the secondarybattery produced using the resulting lithium composite compoundparticles tends to be deteriorated in cycle characteristics. The heattreatment temperature is preferably 600 to 800° C.

The holding time in the heat treatment is preferably 1 to 5 hr. When theholding time is shorter than 1 hr, the crystallinity of the surface ofthe respective particles tends to be insufficient. When the holding timeis longer than 5 hr, the balance between productivity and costs tends tobecome deteriorated.

The atmosphere used in the heat treatment is either air having acarbonate concentration of not more than 100 ppm or oxygen having acarbonate concentration of not more than 100 ppm. When the carbonateconcentration in the atmosphere is more than 100 ppm, the secondarybattery produced using the resulting lithium composite compoundparticles tends to be deteriorated in cycle characteristics. Inaddition, in a reducing atmosphere such as nitrogen, release of oxygentends to be undesirably caused upon the heat treatment.

According to the above production process, it is possible to attain theionic strength ratio A (LiO⁻/NiO₂ ⁻), the ionic strength ratio B (Li₃CO₃⁺/Ni⁺), the powder pH value, the carbon content, the sulfur content, theionic strength ratio C (LiSO₃ ⁻/NiO₂ ⁻), the sodium content, the lithiumcarbonate component content and the lithium hydroxide content as definedin the present invention.

Next, a positive electrode using the positive electrode active substancecomprising the lithium composite compound particles according to thepresent invention is described.

When producing the positive electrode using the positive electrodeactive substance according to the present invention, a conducting agentand a binder are added to the lithium composite compound particles by anordinary method. Examples of the preferred conducting agent includeacetylene black, carbon black and graphite. Examples of the preferredbinder include polytetrafluoroethylene and polyvinylidene fluoride.

The secondary battery produced by using the positive electrode activesubstance according to the present invention comprises the abovepositive electrode, a negative electrode (anode) and an electrolyte.

Examples of a negative electrode (anode) active substance which may beused for production of the negative electrode include metallic lithium,lithium/aluminum alloys, lithium/tin alloys, and graphite or black lead.

Also, as a solvent for the electrolyte solution, there may be usedcombination of ethylene carbonate and diethyl carbonate, as well as anorganic solvent comprising at least one compound selected from the groupconsisting of carbonates such as propylene carbonate and dimethylcarbonate, and ethers such as dimethoxyethane.

Further, as the electrolyte, there may be used a solution prepared bydissolving lithium phosphate hexafluoride or otherwise at least onelithium salt selected from the group consisting of lithium perchlorateand lithium borate tetrafluoride in the above solvent.

<Function>

In order to improve characteristics of the secondary battery, it isimportant to suppress deterioration or degradation of the surface of thelithium composite compound particles forming the positive electrodeactive substance thereof. In particular, in order to improve thehigh-temperature storage property of the secondary battery, etc., it isimportant how to suppress generation of gasses within the secondarybattery.

The impurities being present within the secondary battery tend to havean adverse influence on various characteristics thereof. In particular,the lithium raw materials added in an excess amount upon the reactionand synthesis of the lithium composite compound particles tend to remainin an unreacted state on the surface of the resulting particles tothereby cause a further adverse influence upon producing the secondarybattery. In addition, if lithium oxide and lithium hydroxide act as astrong alkali, gelation of the particles upon formation of a coatingmaterial tends to occur, or the coating material tends to bedeteriorated in storage property. Also, if lithium carbonate is formed,generation of gases tends to occur upon charging within the secondarybattery, thereby exerting an adverse influence on cycle characteristicsand storage property of the resulting secondary battery. Further, iflithium is present in the form of lithium sulfate on the surface of therespective particles, the resulting secondary battery tends to sufferfrom increase in impedance upon storage and as a result, tends to bedeteriorated in cycle characteristics.

In order to suppress occurrence of the above drawbacks, it is requiredthat residual amounts of the surplus lithium, the sulfate component andthe carbonate component in the particles are reduced to as small a levelas possible.

However, the nickel-based positive electrode active substance such asLiNiO₂ tends to undergo release of lithium even from an inside of theparticles when contacted with water, thereby starting breakage of acrystal structure from the surface of the respective particles.

Under these circumstances, in the present invention, in order tosuppress occurrence of the above phenomenon, the particles are washed tosuch an extent as to reduce only a surplus amount of the respectivecomponents, and subjected to heat treatment in an atmosphere ofdecarbonated air or decarbonated oxygen to control surface properties ofthe particles. As a result, it is possible to obtain the lithiumcomposite compound particles which have a less content of residual saltssuch as the surplus lithium and are well controlled in specific surfacearea and crystallinity.

If the washing conditions are inadequate, components being present onthe surface of the respective particles tend to be partially formed intolow-valence components which tend to be dissolved out upon charging anddischarging and deposited on the negative electrode, so that there tendsto arise such a problem that the thus deposited components tend to actas a resistance component upon desorption and insertion of Li, or cyclecharacteristics of the resulting battery tend to be deteriorated. Tosolve these problems, according to the present invention, the surfaceconditions of the lithium composite compound particles are suitablycontrolled by measuring ionic strengths of impurities thereon using atime-of-flight secondary ion mass spectrometer (TOF-SIMS), reducing theamounts of the impurities being present on the surface of the respectiveparticles, and stabilizing a crystallinity of the surface of therespective particles. As a result, it is possible to provide a positiveelectrode active substance for a secondary battery which has good cyclecharacteristics and an excellent high-temperature storage property.

EXAMPLES

Typical examples of the present invention are described in more detailbelow.

The obtained product was identified using a powder X-ray diffractometer(manufactured by RIGAKU Co., Ltd.; Cu—Kα; 40 kV; 40 mA).

The elemental analysis was carried out using a plasma emissionspectroscopic device (“SPS 4000” manufactured by Seiko Denshi Kogyo Co.,Ltd.).

The average primary particle diameter of the particles was determinedusing a scanning electron microscope “SEM-EDX” equipped with an energydisperse type X-ray analyzer (manufactured by Hitachi High-TechnologiesCorp.).

The average secondary particle diameter (D50) of the particles wasexpressed by a volume-median particle diameter as measured by a wetlaser method using a laser type particle size distribution measuringapparatus “LMS-30” manufactured by Seishin Kigyo Co., Ltd.

The condition of existence of the particles which are coated or allowedto be present on core particles was observed using a scanning electronmicroscope “SEM-EDX” equipped with an energy disperse type X-rayanalyzer (manufactured by Hitachi High-Technologies Corp.) and atime-of-flight secondary ion mass spectrometer “TOF-SIMS5” (manufacturedby ION-TOF Inc.) to calculate an ionic strength ratio A (LiO⁻/NiO₂ ⁻),an ionic strength ratio B (Li₃CO₃ ⁺/Ni⁺) and an ionic strength ratio C(LiSO₃ ⁻/NiO₂ ⁻) thereof.

The powder pH value of the particles was determined as follows. That is,0.5 g of the particles was suspended in 25 mL of distilled water toprepare a 2 wt % dispersion, and then the resulting dispersion wasallowed to stand at room temperature to measure a pH value thereof.

The carbon content of the particles was determined as follows. That is,a sample was burnt in an oxygen flow in a combustion furnace to measurea carbon content thereof using a carbon/sulfur measuring apparatus“EMIA-520” manufactured by Horiba Seisakusho Co., Ltd.

The sulfur content of the particles was determined as follows. That is,a sample was burnt in an oxygen flow in a combustion furnace to measurea sulfur content thereof using a carbon/sulfur measuring apparatus“EMIA-520” manufactured by Horiba Seisakusho Co., Ltd.

The sodium content of the particles was determined using a plasmaemission spectroscopic device (“SPS 4000” manufactured by Seiko DenshiKogyo Co., Ltd.).

The contents of a lithium carbonate component and lithium hydroxide weredetermined as follow. That is, 20 g of a sample were suspended in 100 mLof deionized water in a conical flask, and after hermetically sealingthe flask with in an argon (Ar) atmosphere, the resulting suspension wasstirred for 20 min using a magnetic stirrer to extract a surplus amountof lithium carbonate and lithium hydroxide in a solvent. The obtainedextract was subjected to suction filtration to separate the extract intothe sample and a filtrate. The thus obtained filtrate was subjected totitration using hydrochloric acid. The terminal point of the titrationwas determined using phenolphthalein and Bromocresol Green Methyl asindicators, and the amounts of lithium carbonate and lithium hydroxidein the sample were estimated from titers to determine surplus amounts ofthe respective components.

The BET specific surface area of the particles was measured by BETmethod using nitrogen.

The battery characteristics of the positive electrode active substancewere determined as follows. That is, the positive electrode, negativeelectrode and electrolyte solution were produced by the followingmethods, and a coin cell was produced therefrom to evaluate the batterycharacteristics of the positive electrode active substance.

<Production of Positive Electrode>

The positive electrode active substance, acetylene black as a conductingagent and polyvinylidene fluoride as a binder were accurately weighedsuch that a weight ratio therebetween was 85:10:5, and fully mixed witheach other in a mortar. Then, the resulting mixture was dispersed inN-methyl-2-pyrrolidone to prepare a positive electrode preparationslurry. Next, the thus prepared slurry was applied on an aluminum foilas a current collector to form a coating layer having a thickness of 150μm, and dried in vacuum at 150° C. The thus obtained coated foil wasblanked into a disk shape having a diameter of 16 mmφ to produce apositive electrode plate.

<Production of Negative Electrode>

A metallic lithium foil was blanked into a disk shape having a diameterof 16 mmφ to produce a negative electrode.

<Preparation of Electrolyte Solution>

A mixed solution was prepared by mixing ethylene carbonate and diethylcarbonate with each other at a volume ratio of 50:50, and 1 mol/L oflithium phosphate hexafluoride (LiPF₆) was mixed in the mixed solutionto prepare an electrolyte solution.

<Assembly of Coin Cell>

In a glove box placed in an argon atmosphere, the above positiveelectrode and negative electrode were disposed in a SUS316 casingthrough a polypropylene separator, and the electrolyte solution wasfilled in the casing to produce a coil cell of CR2032 type.

<Evaluation of Battery>

The coin cell thus produced was subjected to charge/discharge test forsecondary batteries. The measuring conditions were as follows. That is,the coin cell was repeatedly subjected to charging and dischargingcycles at rate of 1.0 C at a cut-off voltage between 3.0 V and 4.3 V.The charging and discharging cycle at a rate of 1.0 C is completed for ashort period of time as compared to the charging and discharging cycleat a rate of 0.2 C, etc., (the cycle time at 1 C is 1 hr whereas thecycle time at 0.2 C is 5 hr), i.e., the charging and discharging cycleat a rate of 1.0 C is carried out at a large current density.

Swelling of the battery was determined as follows. That is, a 500 mAhlaminated type cell was produced using a carbon negative electrode. Thecell was charged until reaching 4.2 V and stored at 85° C. for 24 hr tomeasure volumes of the coin cell before and after being stored andcalculate a rate of change in volume therebetween.

The increase in resistance of the coin cell was determined as follows.That is, the coin cell was charged until reaching 4.3 V and stored at60° C. for 4 weeks to measure AC impedance values before and after beingstored and calculate a rate of increase in resistance of the cell. Themeasurement of the impedance values was carried out using an ACimpedance measuring device constructed of a 1287-type interface and a1252A type frequency response analyzer both manufactured by SolartronCo., Ltd.

Example 1

A hydroxide of cobalt, nickel and aluminum was mixed with lithiumhydroxide at such a mixing ratio that a molar ratio of Li/(Ni+Co+Al) was1.08, and the resulting mixture was calcined at 750° C. in an oxygenatmosphere for 20 hr to thereby obtain lithium composite compoundparticles. The thus obtained lithium composite compound particles weredeaggregated, and 60 g of the deaggregated particles were suspended in300 mL of deionized water maintained at a water temperature of 10° C.The resulting suspension was stirred for 20 min and then subjected tofiltration and washing.

The obtained particles were dried at 120° C. over one night,deaggregated again and then subjected to heat treatment in adecarbonated oxygen atmosphere (CO₂ concentration: 20 ppm) at 850° C.for 2 hr.

The thus obtained lithium composite compound particles were evaluatedusing a time-of-flight secondary ion mass spectrometer. As a result, itwas confirmed that the lithium composite compound particles had an ionicstrength ratio A (LiO⁻/NiO₂ ⁻) of 0.04, an ionic strength ratio B(Li₃CO₃ ⁺/Ni⁺) of 3.8 and an ionic strength ratio C (LiSO₃ ⁻/NiO₂ ⁻) of0.07. The obtained particles were embedded in a resin and then subjectedto FIB processing. As a result of observing and analyzing a near-surfaceportion (FIG. 3) and an inside portion (FIG. 2) of the thusresin-embedded particles by electron diffraction, it was confirmed thatany of the portions had a diffraction pattern belonging to R-3m having ahigh crystallinity.

Example 2

The respective raw materials were mixed with each other at such a mixingratio that a molar ratio of Li/(Ni+Co+Al) was 1.02, and the resultingmixture was subsequently treated in the same manner as defined inExample 1 to thereby obtain particles. The thus obtained particles werewashed, dried and then subjected to heat treatment in a decarbonatedoxygen atmosphere (CO₂ concentration: 20 ppm) at 800° C. for 2 hr.

Example 3

The respective raw materials were mixed with each other at such a mixingratio that a molar ratio of Li/(Ni+Co+Al) was 1.02, and the resultingmixture was subsequently treated in the same manner as defined inExample 1 to thereby obtain particles. The thus obtained particles werewashed, dried and then subjected to heat treatment in a decarbonated airatmosphere (CO₂ concentration: 20 ppm) at 800° C. for 2 hr.

Example 4

A hydroxide of cobalt, nickel and aluminum was mixed with boric acid andlithium hydroxide at such a mixing ratio that a molar ratio ofLi/(Ni+Co+Al+B) was 1.07, and the resulting mixture was calcined at 750°C. in an oxygen atmosphere for 20 hr to thereby obtain lithium compositecompound particles. The thus obtained lithium composite compoundparticles were deaggregated, and 60 g of the deaggregated particles werewashed in the same manner as defined in Example 1.

The obtained particles were dried at 120° C. over one night,deaggregated again and then subjected to heat treatment in adecarbonated oxygen atmosphere (CO₂ concentration: 20 ppm) at 500° C.for 2 hr.

Example 5

The respective raw materials were mixed with each other at such a mixingratio that a molar ratio of Li/(Ni+Co+Al+B) was 1.10, and the resultingmixture was subsequently treated in the same manner as defined inExample 4 to thereby obtain particles. The thus obtained particles werewashed, dried and then subjected to heat treatment in a decarbonated airatmosphere (CO₂ concentration: 20 ppm) at 700° C. for 2 hr.

Example 6

The respective raw materials were mixed with each other at such a mixingratio that a molar ratio of Li/(Ni+Co+Al+B) was 1.10, and the resultingmixture was subsequently treated in the same manner as defined inExample 4 to thereby obtain particles. The thus obtained particles werewashed, dried and then subjected to heat treatment in a decarbonatedoxygen atmosphere (CO₂ concentration: 20 ppm) at 600° C. for 2 hr.

Comparative Example 1

The same procedure as defined in Example 1 was conducted except that thelithium composite compound particles obtained by the calcination weresubjected to no washing treatment. The resulting particles were embeddedin a resin and then subjected to FIB processing. As a result ofobserving and analyzing a near-surface portion of the thus obtainedparticles by electron diffraction (FIG. 4), it was confirmed that theportion had a diffraction pattern belonging to R-3m having a lowcrystallinity.

Comparative Example 2

The same procedure as defined in Example 4 was conducted except that thelithium composite compound particles obtained by the calcination weresubjected to no washing treatment.

Comparative Example 3

The particles obtained in Example 1 were subjected to neither washingnor drying treatment, and the thus obtained lithium composite compoundparticles were then subjected to heat treatment in a decarbonated oxygenatmosphere (CO₂ concentration: 20 ppm) at 800° C. for 2 hr.

Comparative Example 4

The particles obtained in Example 2 were subjected to washing and dryingtreatments to obtain lithium composite compound particles, and the thusobtained lithium composite compound particles were then subjected toheat treatment in a decarbonated nitrogen atmosphere (CO₂ concentration:20 ppm) at 600° C. for 2 hr.

Comparative Example 5

The particles obtained in Example 4 were subjected to washing and dryingtreatments to obtain lithium composite compound particles, and the thusobtained lithium composite compound particles were then subjected toheat treatment in a decarbonated oxygen atmosphere (CO₂ concentration:20 ppm) at 300° C. for 2 hr.

Comparative Example 6

The particles obtained in Example 5 were subjected to washing and dryingtreatment to obtain lithium composite compound particles, and the thusobtained lithium composite compound particles were then subjected toheat treatment in a decarbonated nitrogen atmosphere (concentration: 20ppm) at 850° C. for 2 hr.

Comparative Example 7

The particles obtained in Example 5 were subjected to washing and dryingtreatment to obtain lithium composite compound particles, and the thusobtained lithium composite compound particles were then subjected toheat treatment in a non-decarbonated air atmosphere (CO₂ concentration:350 ppm) at 500° C. for 2 hr.

Comparative Example 8

The particles obtained in Example 5 were subjected to washing and dryingtreatment to obtain lithium composite compound particles, and the thusobtained lithium composite compound particles were then subjected toheat treatment in a non-decarbonated oxygen atmosphere (CO₂concentration: 350 ppm) at 800° C. for 2 hr. The resulting particleswere embedded in a resin and then subjected to FIB processing. As aresult of observing and analyzing a near-surface portion of the thusobtained particles by electron diffraction (FIG. 5), it was confirmedthat the portion had a diffraction pattern belonging to R-3m having apolycrystalline-like structure.

The production conditions used in the above Examples and ComparativeExamples are shown in Table 1, and the compositional ratios and variousproperties of the resulting lithium composite compound particles areshown in Table 2 and Table 3, respectively.

TABLE 1 Production conditions Washing Li/metal treatment Exampleselements Washing Heat treatment and ratio with Atmosphere Comparativebefore water Temp. Decarbon- Examples washing (—) (° C.) Kind ationExample 1 1.08 Done 850 Oxygen ◯ Example 2 1.02 Done 800 Oxygen ◯Example 3 1.02 Done 800 Air ◯ Example 4 1.07 Done 500 Oxygen ◯ Example 51.10 Done 700 Air ◯ Example 6 1.10 Done 600 Oxygen ◯ Comparative 1.08None — — — Example 1 Comparative 1.07 None — — — Example 2 Comparative1.08 None 800 Oxygen ◯ Example 3 Comparative 1.02 Done 600 Nitrogen ◯Example 4 Comparative 1.07 Done 300 Oxygen ◯ Example 5 Comparative 1.10Done 850 Nitrogen ◯ Example 6 Comparative 1.10 Done 500 Air X Example 7Comparative 1.10 Done 800 Oxygen X Example 8

TABLE 2 Examples and Compositional ratio ComparativeLi_(1+x)Ni_(1−y−z1−z2)Co_(y)M1_(z1)M2_(z2)O₂ Compositional Examples x yM1 z1 M2 z2 formula Example 1 −0.02 0.15 Al 0.05 — —Li_(0.98)Ni_(0.80)Co_(0.15)Al_(0.05)O₂ Example 2 0.01 0.15 Al 0.05 — —Li_(1.01)Ni_(0.80)Co_(0.15)Al_(0.05)O₂ Example 3 0.01 0.15 Al 0.05 — —Li_(1.01)Ni_(0.80)Co_(0.15)Al_(0.05)O₂ Example 4 0.00 0.15 Al 0.04 B0.01 Li_(1.00)Ni_(0.80)Co_(0.15)Al_(0.04)B_(0.01)O₂ Example 5 −0.01 0.15Al 0.04 B 0.01 Li_(0.99)Ni_(0.80)Co_(0.15)Al_(0.04)B_(0.01)O₂ Example 60.02 0.15 Al 0.04 B 0.01 Li_(1.02)Ni_(0.80)Co_(0.15)Al_(0.04)B_(0.01)O₂Comparative 0.08 0.15 Al 0.05 — — Li_(1.08)Ni_(0.80)Co_(0.15)Al_(0.05)O₂Example 1 Comparative 0.07 0.15 Al 0.04 B 0.01Li_(1.07)Ni_(0.80)Co_(0.15)Al_(0.04)B_(0.01)O₂ Example 2 Comparative0.07 0.15 Al 0.05 — — Li_(1.07)Ni_(0.80)Co_(0.15)Al_(0.05)O₂ Example 3Comparative 0.00 0.15 Al 0.05 — — Li_(1.00)Ni_(0.80)Co_(0.15)Al_(0.05)O₂Example 4 Comparative 0.04 0.15 Al 0.04 B 0.01Li_(1.04)Ni_(0.80)Co_(0.15)Al_(0.04)B_(0.01)O₂ Example 6 Comparative0.03 0.15 Al 0.04 B 0.01 Li_(1.03)Ni_(0.80)Co_(0.15)Al_(0.04)B_(0.01)O₂Example 7 Comparative 0.02 0.15 Al 0.04 B 0.01Li_(1.02)Ni_(0.80)Co_(0.15)Al_(0.04)B_(0.01)O₂ Example 8 Comparative0.02 0.15 Al 0.04 B 0.01 Li_(1.02)Ni_(0.80)Co_(0.15)Al_(0.04)B_(0.01)O₂Example 9

TABLE 3 Surface condition data:ionic strength ratios Examples andTOF-SIMS Comparative LiO⁻/NiO₂ ⁻ Li₃CO₃ ⁺/Ni⁺ LiSO₃ ⁻/NiO₂ ⁻ Examples(—) (—) (—) Example 1 0.04 3.8 0.07 Example 2 0.14 12.9 0.03 Example 30.18 16.4 0.05 Example 4 0.14 7.3 0.04 Example 5 0.07 8.2 0.07 Example 60.22 19.4 0.06 Comparative 2.45 103.4 1.05 Example 1 Comparative 1.37201.7 0.55 Example 2 Comparative 2.25 98.2 1.04 Example 3 Comparative0.37 12.4 0.13 Example 4 Comparative 0.52 25.8 0.12 Example 6Comparative 0.52 22.1 0.04 Example 7 Comparative 0.19 44.3 0.07 Example8 Comparative 0.28 67.3 0.05 Example 9 Data of particles Surpluscomponents Examples and Properties Carbon Comparative D50 BET Powder pHcontent Examples (μm) (m²/g) (—) (ppm) Example 1 13.1 0.07 10.5 185Example 2 12.6 0.11 10.7 218 Example 3 13.0 0.08 10.7 194 Example 4 12.50.52 10.7 106 Example 5 12.7 0.16 10.6 129 Example 6 12.6 0.42 10.8 181Comparative 12.8 0.12 11.4 451 Example 1 Comparative 12.2 1.33 11.2 380Example 2 Comparative 12.9 0.09 11.3 435 Example 3 Comparative 12.5 0.3310.6 188 Example 4 Comparative 12.4 0.87 10.8 240 Example 6 Comparative12.5 0.09 10.8 265 Example 7 Comparative 12.5 0.63 10.9 380 Example 8Comparative 12.3 0.11 10.9 257 Example 9 Data of particles Surpluscomponents Examples and Sulfur Sodium Comparative content content LiOHLi₂CO₃ Examples (ppm) (ppm) (wt %) (wt %) Example 1 40 <5 0.09 0.10Example 2 10 <5 0.18 0.16 Example 3 18 <5 0.17 0.16 Example 4 5 12 0.130.10 Example 5 7 <5 0.14 0.11 Example 6 9 <5 0.19 0.17 Comparative 703231 0.40 0.31 Example 1 Comparative 530 320 0.41 0.33 Example 2Comparative 701 240 0.41 0.30 Example 3 Comparative 62 <5 0.17 0.15Example 4 Comparative 45 12 0.34 0.33 Example 6 Comparative <5 <5 0.280.22 Example 7 Comparative <5 <5 0.33 0.31 Example 8 Comparative <5 <50.24 0.28 Example 9 Battery evaluation data Cycle Storage propertyExamples and characteristics Increase in Comparative Retention rateSwelling resistance Examples (%) (%) (%) Example 1 97 15 40 Example 2 9815 45 Example 3 97 17 65 Example 4 98 18 45 Example 5 97 10 30 Example 698 10 35 Comparative 91 50 185 Example 1 Comparative 92 45 170 Example 2Comparative 92 48 150 Example 3 Comparative 85 22 40 Example 4Comparative 94 50 115 Example 6 Comparative 82 24 65 Example 7Comparative 85 44 95 Example 8 Comparative 86 47 95 Example 9

The resulting lithium composite compound particles (Example 1 andComparative Examples 1 and 8) were respectively embedded in a resin andthen subjected to FIB processing. Thereafter, the lithium compositecompound particles were subjected to nano-ED (electron diffraction) asshown in FIG. 1 to determine the conditions of a near-surface portion (Bin FIG. 1) and an inside portion (A in FIG. 1) of the particles. As aresult, it was confirmed that a central portion of any of the sampleparticles maintained a good crystallinity (FIG. 2).

As shown in FIG. 4, it was expected that the untreated particles(Comparative Example 1) had a poor crystallinity on the surface thereofso that movement of lithium therein was inhibited. Also, as shown inFIG. 5, it was confirmed that the particles subjected to the heattreatment but treated in a non-decarbonated atmosphere (ComparativeExample 8) had a polycrystalline-like structure although they wereimproved in crystallinity as compared to those of Comparative Example 1.In addition, as shown in FIG. 3, it was confirmed that the particlessubjected to the heat treatment in a decarbonated atmosphere (Example 1)were improved in crystallinity, and the battery produced using theparticles were also improved in cycle characteristics.

Thus, the secondary batteries produced using the lithium compositecompound particles according to the present invention had good batterycharacteristics such as a cycle retention rate of not less than 95%, andgood storage properties such as a cell swelling rate as low as not morethan 20% and further a rate of increase in resistance as low as not morethan 70%.

INDUSTRIAL APPLICABILITY

The lithium composite compound particles according to the presentinvention are excellent in cycle characteristics and high-temperaturestorage property required as a positive electrode active substance forsecondary batteries, and can be therefore suitably used as the positiveelectrode active substance for secondary batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph showing an observation position of asection of respective lithium composite compound particles according tothe present invention in which A indicates a central portion of theparticles whereas B indicates a surface portion of the particles.

FIG. 2 is an electron diffraction microphotograph showing a centralportion of the lithium composite compound particles obtained in Example1.

FIG. 3 is an electron diffraction microphotograph showing a surfaceportion of the lithium composite compound particles obtained in Example1.

FIG. 4 is an electron diffraction microphotograph showing a surfaceportion of the lithium composite compound particles obtained inComparative Example 1.

FIG. 5 is an electron diffraction microphotograph showing a surfaceportion of the lithium composite compound particles obtained inComparative Example 8.

1. Lithium composite compound particles comprising a lithium compositecompound represented by the following compositional formula 1, whichlithium composite compound particles have an ionic strength ratio A(LiO⁻/NiO₂ ⁻) of not more than 0.3 and an ionic strength ratio B (Li₃CO₃⁺/Ni⁺) of not more than 20 as measured on a surface of the respectivelithium composite compound particles using a time-of-flight secondaryion mass spectrometer:Li_(1+x)Ni_(1-y-z)Co_(y)M_(z)O₂  Compositional formula 1 wherein M is atleast one element selected from the group consisting of B and Al; and x,y and z satisfy −0.02≦x≦0.02, 0<y≦0.20 and 0<z≦0.10, respectively. 2.Lithium composite compound particles according to claim 1, wherein thelithium composite compound particles have an average secondary particlediameter of 1 to 30 μm.
 3. Lithium composite compound particlesaccording to claim 1, wherein the lithium composite compound particleshave a powder pH value of not more than 11.0 as measured in a 2% byweight suspension prepared by dispersing the lithium composite compoundparticles in water.
 4. Lithium composite compound particles according toclaim 1, wherein the lithium composite compound particles have a carboncontent of not more than 300 ppm.
 5. Lithium composite compoundparticles according to claim 1, wherein the lithium composite compoundparticles have a sulfur content of not more than 100 ppm, an ionicstrength ratio C (LiSO₃ ⁻/NiO₂ ⁻) of not more than 0.3 and a sodiumcontent of not more than 100 ppm.
 6. Lithium composite compoundparticles according to claim 1, wherein the lithium composite compoundparticles have a lithium carbonate component content of not more than0.30% by weight and a lithium hydroxide content of not more than 0.30%by weight.
 7. Lithium composite compound particles according to claim 1,wherein the lithium composite compound particles have a specific surfacearea of 0.05 to 0.7 m²/g.
 8. A process for producing the lithiumcomposite compound particles as defined in a, comprising the steps of(1) treating lithium composite compound particles with a water solventto remove impurities therefrom; and (2) subjecting the lithium compositecompound particles obtained in the step (1) to heat treatment, a ratioof a total molar amount of lithium to a total molar amount of atransition element, aluminum and boron in the lithium composite compoundparticles used in the step (1) being not less than 1.02 and not morethan 1.10.
 9. A process according to claim 8, wherein the heat treatmentin the step (2) is conducted in a temperature range of 500 to 850° C. inan air or oxygen atmosphere having a carbonate concentration of not morethan 100 ppm.
 10. A non-aqueous electrolyte secondary battery comprisingthe lithium composite compound particles as defined in claim 1.